In line based communication systems, communication takes place between subscriber stations connected to one another by wires, with switching centers generally being interposed between these subscriber stations.
In radio communication systems, for example in the second generation European mobile radio system GSM (Global System for Mobile Communications), information such as speech, image information or other data is transmitted via a radio interface using electromagnetic waves. The radio interface refers to a connection between a base station and a large number of subscriber stations, the subscriber stations being able to be mobile stations or fixed radio stations, for example. In this case, the electromagnetic waves are radiated at carrier frequencies which are in a frequency band provided for the respective system. For future radio communication systems, for example the UMTS (Universal Mobile Telecommunication System) or other 3rd generation systems, frequencies in a frequency band around approximately 2000 MHz are provided. For the third mobile radio generation UMTS, two modes are provided, one mode denoting FDD (Frequency Division Duplex) operation and the other mode denoting TDD (Time Division Duplex) operation. These modes are used in different frequency bands, with both modes supporting a code division “CDMA” (Code Division Multiple Access) subscriber separation method.
For data access, particularly in line based systems, connections between a subscriber station and an access network are normally controlled using a point-to-point protocol (PPP). When setting up a connection, the subscriber or the subscriber station setting up the connection is authenticated, which, inter alia, can be the basis for central billing of charges incurred. For the purposes of authentication, the RADIUS (Remote Authentication Dial In User Service) protocol is known.
If such communication systems provide the subscriber stations, e.g. a computer or host, with network access for data transmission, e.g. for Internet services, then installation of a subscriber station involves configuration, where all settings, particularly IP (Internet Protocol) addresses, required for data access are stored in the subscriber station. These allow the subscriber station, or the subscriber, to contact supporting services which are absolutely necessary for the operating sequence of the desired data service, e.g. Internet access, and also network components on which these services run. These addresses include not only the IP address of an HTTP server which allows an Internet page to be called up conveniently “by mouse click”, for example, but also, inter alia, the address of a DNS (Domain Name Server), which ascertains the IP address associated with a subscriber, and also the address of an SMTP (Simple Mail Transfer Protocol) server, which is necessary for transmitting electronic messages, known as e-mail.
Besides the communication systems listed above, there are data networks which generally have a local structure and are designed for universally connecting data stations, unreservedly referred to as hosts below merely in order to distinguish them from the subscriber stations referred to above. Two host computers can be connected to one another directly or via hubs and bridges and to network devices, such as an access server. Data packets are transported between a host and another network device normally by the IP (Internet Protocol).
When connecting a host to a network, for example a local area data network (LAN), a connection or access server uses, by way of example, the “dynamic host configuration protocol” (DHCP) to allocate the host an IP address at which the host can clearly be identified and addressed in the network. In addition, the host is in this case notified of addresses for auxiliary services of importance to it and for network components on which these services are executed in this network.
A host is normally connected to a local area network by wires, with the last part of wire based access in relatively new networks being able to be effected wirelessly by radio. A local area network which supports wireless connection of hosts is referred to universally as a W-LAN (Wireless Local Area Network).
When a connection is set up, the host setting up the connection is authenticated and authorized, if at all, on an authentication and authorization server which can optionally also permit central billing of charges incurred (AAA server).
The communication systems and networks described above thus differ in terms of a large number of features, which means that it is currently not possible to connect stations in one system to those in another system directly.
Upon installation for IP access, a subscriber station in a telecommunication system needs to be notified, by software installation, of the IP addresses required for connection setup in advance before initial connection or connection setup to a corresponding network interface. By contrast, in a local area data network, all IP addresses required are automatically assigned directly whenever a host reconnects to a data network. A subscriber station and a host are thus not devices which are compatible with the other network technology.
Another example of incompatibility can be seen in the type of connection setup, for example. On the one hand, a point-to-point connection protocol (PPP) is used between subscriber stations and the network-end data terminals, known as RASs (Remote Access Server), in the telecommunication systems, the point-to-point protocol being used between the actual, network specific transport protocol layers and the IP for the purpose of connection control. On the other hand, in local area networks, known as LANs, inter alia, this additional connection control is not necessary, however, which means that data packets, “IP packets”, can be transmitted directly on the underlying transport layer. The transport layer is advantageously provided by an Ethernet.
Particularly in a radio telecommunication system, the system's visitor location register (VLR) and the subscriber home location register (HLR) additionally always need to be informed about the current location of an active subscriber and of his service access entitlements. Otherwise, it would not be possible to start setting up a connection to this subscriber from another subscriber station. In contrast to this, comparable functions of a visitor location register do not need to be available, or need to be available only to a limited extent, in a local area data network, since the network clients are pure retrieval services sending a triplet to the network, for example. A triplet comprises two challenge values and a value for an expected response (response value).
Each of the communication technologies has its own specific advantages and drawbacks. By way of example,                wired access by a station to the corresponding telecommunication system is fast but fixed, i.e. tied to one location,        although point-to-point access is connection oriented, it is not freely configurable,        cellular access (radio telecommunication system) is largely mobile but is limited in terms of transmission bandwidth,        access to a local area network (LAN) is self-configurable or auto-configurable but unsafe, since there is generally no authentication.        
In different technologies, there are approaches or considerations regarding integration of other technologies' advantages into the technology in question:                in the case of IETF (Internet Engineering Task Force), extensions to existing protocols and/or new protocols are planned,        in the case of UMTS, provision needs to be made for the transmission of speech (voice) and various data services,        in the case of GSM, a special packet data service (GPRS: General Packet Radio Service) is being introduced, . . .        
In this case, however, aspects of other technologies are disadvantageously always integrated by a specific and complex extension of the technology in question.
The currently available data network technologies allow a subscriber to use a notebook having a radio data network card, e.g. at an airport, to register in a foreign data network which can be accessed there using a radio interface. This is possible because open-plan data networks do not involve any authorization check being performed. However, the network operator can prevent the foreign subscriber from accessing particular files or programs in the data network only if he uses a high level of programming complexity.
In this case, the programming needs to be done in various devices in the data network and in the various hosts which are to be protected from foreign access. Only a limited amount of protection is possible, particularly when the host or the subscriber has knowledge of network-internal IP addresses.
EP 0766427 A2 describes an office communication system in which data are interchanged between a local area network (LAN) and a GSM network. To this end, the local area network has an interface (gateway computer) which converts the local area network's data into a data format for the GSM network, and vice versa.